1. Field of the Invention
This invention relates to a wet-type friction material and its manufacturing method that is able to stabilize an initial frictional property of the wet-type friction material without using a polishing device or a cutting device, thereby shortening a manufacturing time to a large extent, while preventing unnecessary polishing dusts or cutting dusts from being generated.
2. Description of the Related Art
Commonly used wet-type friction material for an automatic transmission is composed of a core metal of a flat ring plate shape and a friction material substrate. The friction material substrate is made of a ring-shaped friction material substrate sheet (may be referred to as “ring sheet” hereafter) or a plurality of segmented friction material sheets (may be referred to as “segment pieces” hereafter). Either of the ring sheet and the segment pieces is cut out from a friction material substrate as a base material sheet. Then, the ring sheet is adhered to one surface of the core metal or two ring sheets are adhered to both surface of the core metal. Alternatively, a group of the segment pieces is adhered to one surface of the core metal in a ring arrangement or two groups of the segment pieces are adhered to both surfaces of the core metal. Each of these wet-type friction material has a rotating lining surface (surface of frictional material substrate) pressed to a disc or the like to transmit a rotational force. Therefore, it is necessary that a frictional coefficient of the lining surface is stable. In view of that, as shown in a first patent publication (Japanese Patent Publication No. H6-37900), an initial smoothing is operated on a newly produced wet-type friction material in a manufacturing process in order to make the frictional coefficient constant.
However, the initial smoothing operation of the wet-type friction material is conducted, while the wet-type friction material and a disc being assembled in a device similar to an actual device as a final product. Consequently, it takes much time to obtain a target frictional coefficient and leads to an increase of manufacturing steps and a delay in a production time. As a result, it has been impossible conventionally to attain a reduction in manufacturing costs. In view of that, a second patent publication (Japanese Laid Open Patent Publication No. H11-193841) discloses an improved ring-shaped friction material as a wet-type friction material. In the friction material, a friction surface is made of an abraded surface that is polished by a polishing surface composed of a flat surface and a plurality of grooves shaped on the flat surface.
It is described in the second patent publication that, in case of the wet-type friction material, the friction surface is abraded by the polishing surface composed of the flat surface and the plural grooves made on the flat surface, so that unnecessary polishing is prevented and a rapid initial smoothing operation is possible, thereby shortening the manufacturing time. A third patent publication (Japanese Laid Open Patent Publication No. 2003-231043) discloses another wet-type friction material. It is described in the third patent publication that the wet-type friction material is capable of obtaining a stable frictional property from the beginning of use by cutting or shaving a front surface of a friction material used as a lock-up clutch.
However, in the techniques described in the second and third patent publications, an abrading step or a cutting step is also necessary in the manufacturing process, though it is shorter than the initial smoothing operation. Moreover, another problem is caused that there is produced unnecessary grinding sludge or cutting chips in these grinding step or cutting step.
In view of that, a fourth patent publication (Japanese Laid Open Patent Publication No. 2004-11710) discloses an improved wet-type frictional facing. In wet-type frictional facing has a ring shape and is provided at an outer peripheral side of a piston of a lock-up clutch. The wet-type fictional facing has a thickness reduced from an outer peripheral side to an inner peripheral side in a radial direction thereof or has a tapered surface. Moreover, a density at the outer peripheral side in the radial direction of the wet-type frictional facing is made smaller than the other part. Alternatively, a tapered surface is formed at an outer peripheral side in a radial direction of a fixing surface of the piston on which a wet-type frictional facing is secured so as to be inclined along a frictional surface to which the wet-type frictional facing is faced. An outer peripheral part in a radial direction of the wet-type frictional facing is a tapered surface corresponding to the fixing surface. It is described in the fourth patent publication that, with the above features, μ-V characteristics of the lock-up clutch is improved so as to restrain judder generation at the time of slip control.
However, in the technique described in the fourth patent publication, a strength at the inner peripheral side in the radial direction of the wet-type frictional facing becomes too high, in case a taper is given such that the density at the outer peripheral side in the radial direction of the wet-type frictional facing becomes smaller than the other part. Consequently, it becomes difficult to make an entire surface of the wet-type frictional facing closely contact with the facing frictional surface, even if a pressure rises at the side of the facing frictional surface to which the wet-type frictional facing is oppositely located. Moreover, in case a taper is provided at the outer peripheral side in the radial direction of the fixing surface, there is a problem that the piston as a whole must be manufactured separately from normal pistons.